In a Hard Disk Drive (HDD) system having a Magneto-Resistive (MR) head for reading data from the disk, the output from the MR head takes the form of a small AC data signal superimposed upon a DC bias voltage. The MR head is located very close to the disk, which spins very rapidly. Accordingly, if there are surface imperfections or an extraneous particle lying on the surface of the disk, the MR head can impact the imperfection or particle at high speed. This causes the MR head to heat up rapidly and then to return to its normal operating temperature more slowly. Since the DC bias voltage is sensitive to temperature, such impacts cause bias transients, known as thermal asperities, to appear in the head output signal.
Thus, a thermal asperity is characterized by a rapid step of the DC bias voltage in one direction, followed by a relatively slow decay back to the normal value. The transient typically has a step time on the order of tens of nano seconds and a decay time constant of micro seconds. The step is always in the same direction for a particular circuit, but whether it is positive or negative depends on the architecture of the circuit. The AC data signal superimposed on the DC bias signal is not significantly affected.
In order to be able to recover the AC data signal for the duration of the thermal asperity it is necessary to effectively remove the transient from the DC bias voltage. However, since the bias transient has components in the same frequency range as the AC data signal, it is therefore difficult to separate the AC component from the DC bias using a linear filter. Typical known techniques involve detection of the mid-point DC voltage by using rectifier peak and trough detectors, whose outputs are then combined to provide the mid-point voltage. Transients in the DC bias voltage can then be subtracted from the input signal.
One such circuit is disclosed in U.S. Pat. No. 4,914,398 (Jove) in which the peak and trough detectors are capacitively coupled via a buffer to allow both detectors to follow the initial rapid step in the DC bias voltage level. Their sum is then filtered by a non-linear signal-adaptive filter before being subtracted from the (delayed) input signal. The problem with this technique is that it does not include a mechanism for allowing tracking of the DC mid-point level during the decay back to the normal operating point.